1. Field of the Invention
The invention relates to a system for sorting data from seismic surveys at sea, wherein data relating to geophysical conditions is recorded together with the geographical position continuously during measurements performed with the aid of streamer cables.
2. Description of the Prior Art
For the acquisition of seismic data at sea, it is conventional practice to tow sets of air guns behind a vessel. The air guns emit pulses which are reflected from different layers of the sea bed, and the reflections are received by an acoustic cable and recorded. Simultaneously, the position of the cable is recorded, so that the individual pulses can be coordinated to the locations of the acquisition of data. A method of such position determination by means of angular headings is described, e.g., in U.S. Pat. No. 3,953,827.
When long streamer cables are being towed behind a ship, they will be influenced by wind and current conditions. Therefore, it is also important at all times to know where the towed cable is located, so that navigation of the vessel can to some degree be adapted to the prevailing conditions. The use of angular headings for position determination can be utilized for recording the cable position, and such a method is described, e.g., in U.S. patent application Ser. No. 885,916 (filed Mar. 13, 1978) U.S. Pat. No. 4,231,111.
With such measurements, however, there will in every instance arise a number of sources of error, which make correct recording and plotting of the earth formation difficult.
In the methods utilized previously for the acquisition of seismic data, it has been conventional to gather the data more or less in a linear direction behind a ship, the ship, following a course so as to form a series of adjacent and parallel lines and corresponding lines crosswise of the first lines. Thus, one obtains data along the lines of a grid or network, so it could be said that the data constitute a framework for the grid squares.
The drawbacks of the known method wherein data is collected in lines include that these lines do not have a uniform configuration owing to deviations caused by current conditions, e.g. wind, etc. Therefore, the recording and analysis of the results are also encumbered by sources of error.